Delta Air Lines CEO on Wednesday asked employees to sign up for more unpaid leave with a slump in travel to continue into 2021.

“Our voluntary unpaid leave program will continue to be essential to positioning Delta for the recovery, and we will need participants for the foreseeable future,” Ed Bastian said in a note to staff. “I ask everyone to consider whether a voluntary leave makes sense for you and your family.”

Executives at Delta and other airlines in recent weeks have warned about a slowdown in bookings as Covid-19 infections rose, making it more challenging to trim its cash burn.

The Transportation Security Administration said it screened 501,513 people at U.S. airports on Tuesday, the fewest since July 4 and about a quarter of the 1.9 million people the TSA screened a year ago.